Siblings
The order in which children are born within the family structure has a profound influence on their personality development. That is to say, the parents' expectations and treatment of the oldest child cannot be identical with that of the youngest one and so forth. The conclusion is that each child experiences a different environment within the same family.
Accompanying the obvious physical distinctions between male and female there is a complex biochemical system which not only distinguishes between the sexes in function, but also creates emotional differences which are further differentiated by the roles and expectations of the society in which they live.
Because of its gender and the fact that it necessarily views the environment from a different level of maturation, each child in a family will develop a distinctly individual personality.